Of course, you know that a genuine croissant should be puffy, buttery, and crunchy. It stays that way for the whole day after baking, if you don't have time to eat it. But if you plan on using it the next day, here's my advice on how to make it crispy again. Wrap a fresh croissant neatly in foil, place it in a plastic bag, and leave it in the refrigerator until you need it. Before serving, put it in preheated to 180˚C oven, of course, without foil and a plastic bag, for 3 minutes. When stored this way, the croissant retains its moisture, so it will become crispy again in the oven, but not dry.